1. Field of the Invention
The present invention relates generally to the manufacture of composite materials and more particularly composite materials with fibrous reinforcements and with vitro-ceramic or ceramic matrix obtained by using a solgel process.
The term vitro-ceramic as used herein, means the matrix is maintained at the vitreous state, i.e. having a rehological behavior facilitating shaping of the material so that it is possible to control the degree of ceramization desired.
Composite materials of the vitro-ceramic-ceramic or ceramic-ceramic type are generally used in the aeronautics and spatial fields for applications requiring good resistance at average or high temperatures, i.e. from about 600.degree. to 2500.degree. C. It is a question for example of carbon-carbon brakes, nozzle necks, heat protection materials.
Composite ceramic-ceramic materials may also be used for certain parts of motor vehicle engines such as turbo compressor rotor, heat exchanger, piston, transmission shafts etc.
2. Description of the Prior Art
Numerous authors have sought to prepare such materials.
Thus, an article by S. R. LEVITT "High strength graphite-fibre/lithium alumino-silicate" J. Materials Science, 8 (1973). 793) describes ceramic matrix composite materials of a composition Li.sub.2 OAl.sub.2 O.sub.3 nSiO.sub.2 where n=3, 4, 8 and with unidirectional reinforcement.
Similarly, an article by K. M. PREWO and J. J. BRENNAN "Silicon carbide fiber reinforced glass-ceramic matrix composites exhibiting high strength and toughness" J. Materials Science 17 (1982) 2371) describes ceramic matrix composites of composition Li.sub.2 OAl.sub.2 O.sub.3 4SiO.sub.2 and with silicon carbide reinforcement, whose production requires very high temperatures (1300.degree.-1600.degree. C.) at a pressure of 7 MPa.
In the French patent n.sup.o 2 521 982 the author describes the preparation of a ceramic-ceramic composite material of the alumina-alumina type using alumina fibers and an alumina sol. The materials of the French patent 2 521 982 have a high residual porosity.
All these approaches for preparing composite ceramic materials use two techniques.
A technique in which the fibrous preform is impregnated with a barbotine (solid phases dispersed in a liquid), this preform thus impregnated then being densified by a hot pressing treatment. This technique requires pressing temperatures higher than 1200.degree. C.
A technique in which the fibrous preform is impregnated by a sol-gel process; the sol consists of a colloidal solution and the gel is formed of agglomerates of elementary particles whose size is about 100 .ANG. and which are disposed in a more or less compact way. Such impregnation may, in some cases, be followed by a hot pressing treatment. Such heat treatment, in the case where no pressure is applied, makes it possible to obtain well densified materials. A residual porosity of at least 20% very often appears. In the case where the impregnation is followed by a hot pressing treatment the temperatures used for obtaining a dense ceramic material are higher than 1100.degree. C.
The applicant has perfected a new method of manufacturing composite ceramic matrix materials using a sol-gel process which makes it possible:
to maintain a vitreous phase during the densification step by hot pressing at a temperature much lower than those used up to now and,
to obtain a dense ceramic matrix (residual porosity rate V.sub.P .ltoreq.10% even 5%) in which the ceramization does not require the addition of a nucleating agent such for example as TiO.sub.2, ZrO.sub.2 or P.sub.2 O.sub.5.
The production of composite materials prepared in accordance with the invention from carbon, silicon carbide, alumina and alumino-silica fibers and a sol combines the impregnation technique using a sol-gel process and the densification technique using hot pressing treatment at a temperature less than 1100.degree. C.
It is known that the ceramics obtained by a sol-gel process tend to crystallize at a low temperature, a prejudicial property when it is a question of densifying a fibrous preform insofar as a premature crystallization prevents a material being obtained with low residual porosity. The characteristic of the invention is to have discovered that the introduction of a crystallization retarding agent in the sol makes it possible to maintain the vitreous phase during the material densification step and to obtain a material with low residual porosity without using a nucleating agent.